This invention relates to a torque sensor detecting a shaft torque applied to a rotary shaft as change of magnetic flux density (i.e., strength of magnetic field).
As a conventional technique, Japanese Patent Application Laid-open No. 2003-149062 (FIG. 15) discloses a torque sensor.
This torque sensor is, for example, used for a motor-driven power steering apparatus. As shown in FIG. 11, an input shaft 100 and an output shaft 110 are coaxially connected by a torsion bar 120. A multipolar magnet 130 is fixed to the input shaft 100. One set of magnetic yokes 140, fixed to the output shaft 110, is disposed in a magnetic field generated from the multipolar magnet 130. One set of flux collecting rings 150 collecting magnetic fluxes generated from the magnetic yokes 140. A magnetic sensor 160 detects the density of magnetic flux generated in an air gap between the opposed flux collecting rings 150.
The above-described conventional torque sensor, if it is positioned closely to a car speaker or a comparable magnet member, may generate an erroneous signal due to influence of magnetic field generated from these magnetic member. Hence, according to the above prior art document, the torque sensor is enclosed by a magnetic material 170 serving as a magnetic shield to eliminate erroneous detection caused by the adverse influences of the geomagnetism or the magnetic field generated around the torque sensor.
However, according to the magnetic shield disclosed in the above-described prior art document, a magnetic circuit of the torque sensor is entirely enclosed by the magnetic material 170. The outer diameter of the torque sensor becomes large and accordingly installability of the torque sensor to an automotive vehicle is worsened. In other words, the above-described conventional torque sensor is not practically applicable for mass production. Furthermore, it is not clear how this torque sensor is installed.